This invention relates to angioplasty using radiation is generated by one or more integrated circuits located at the distal end of the catheter.
Many people die suddenly in the United States each year from acute myocardial infarction, and many more suffer from chronic heart problems. A major contributing factor in both acute and chronic heart problems is a reduction in the flow of nutrient blood to the muscles of the heart, resulting from a reduction of blood flow through coronary blood vessels. The reduction in flow may be caused by deposits of atherosclerotic plaque on the walls of the blood vessel, which causes a narrowing of the lumen or channel of the blood vessel. When the lumen is sufficiently narrowed, the rate of flow of blood may be so diminished that spontaneous formation of a thrombus or clot occurs by a variety of physiological mechanisms. As is known, once a blood clot has started to develop, it extends within minutes into the surrounding blood, in part because the proteolytic action of thrombin acts on prothrombin normally present, tending to split this into additional thrombin which causes additional clotting. Thus, the presence of atherosclerotic plaque not only reduces the blood flow to the heart muscle which it nourishes, but is a major predisposing factor in coronary thrombosis.
Among the treatments available for the conditions resulting from plaque formations are pharmacological means such as the use of drugs, for example nitroglycerin, for dilating the coronary blood vessels to improve flow. In those cases too far advanced to be manageable by drugs, surgical treatment may be indicated. One of the surgical techniques commonly used is the coronary bypass, in which the substitute blood vessel shunts or bypasses blood around the blockage. The bypass operation is effective but is expensive and subject to substantial risks.
Percutaneous transluminal balloon catheter angioplasty is an alternative form of treatment. This method involves insertion of a deflated balloon into the lumen of an artery partially obstructed by plaque, and inflation of the balloon in order to enlarge the lumen. The lumen remains expanded after removal of the catheter. The major problem with this technique is restenosis of the narrowed vessel by recurrence of the arterial plaque.
Another technique which has recently received a great deal of attention is transluminal laser catheter angioplasty. This treatment involves introduction into the coronary artery of a fiber optic cable, the proximal end of which is connected to a laser energy source. The distal end of the fiber optic cable is directed towards the plaque. The laser is pulsed, and the resulting high energy light traverses the fiber optic cable and exits from the distal end thereof to penetrate or vaporize a portion of the plaque. Many problems remain unsolved in laser catheter angioplasty. Among the problems are the difficulty in matching the characteristics of available lasers with the characteristics of fiber optic cables. In particular, the wavelengths at which fiber optic cables having mechanical properties suiTABLE for use in catheters carry light energy with low losses do not necessarily correspond to the wavelengths at which lasers radiate the maximum energy. Consequently, a substantial amount of light energy produced by a laser may not be transmitted or may be absorbed by the fiber optic cable extending throughout the length of the catheter. Furthermore, the connectors by which light is coupled from a laser source to the proximal end of the fiber optic cable of the catheter may introduce attenuation. Thus, the exact amount of light energy or power arriving at the distal end of the fiber optic cable extending through the catheter may not be known.
Microwave aided balloon angioplasty is described in U.S. Pat. No. 4,643,186 issued Feb. 7, 1987 in the name of Rosen et al. In the arrangement as described by Rosen et al., a catheter including a microwave transmission line terminates at its distal end in an antenna. The antenna is surrounded by a balloon. During angioplasty, the catheter is introduced into a blood vessel or other vas, and the distal end with the balloon and the antenna is manipulated to a point adjacent the plaque. Microwave power is applied to the proximal end of the catheter and flows to the antenna, which radiates electromagnetic energy. The electromagnetic energy penetrates the soft tissue and the plaque for heating and thereby softening the plaque. The balloon is expanded against the softened plaque to thereby expand the lumen of the blood vessel. It should be noted that there is no difference in kind between the light radiated by the laser and the microwave power which is radiated by an antenna, both being oscillatory electromagnetic radiation, with different frequencies of oscillation.
A microwave transmission line of the coaxial type which is described in the aforementioned Rosen et al. arrangement may advantageously have certain ratios of the diameters of the center and outer conductors in order to transmit energy most effectively. As is known to those skilled in the art, for maximum power the characteristic impedance of a coaxial transmission line must be in the vicinity of 50 ohms, whereas for lowest loss the characteristic impedance should be near 70 ohms. The very small size of the cable tends to result in relatively large losses. These losses may be so great that up to half the microwave power applied to the proximal end of the transmission line may not reach the distal end for application to the tissue. Instead, this power is dissipated in the form of heat, which tends to be greatest at the proximal end. Since the heat is concentrated within the relatively small transmission line rather than being distributed throughout a larger volume of flesh, the transmission line may become hot enough to cause burns at the point at which the catheter enters the body. Furthermore, the cross-sectional shape of the microwave transmission line makes it difficult to add other functions such as fiber optic scopes and the like. In particular, since the center conductor of a coaxial transmission line is desirably in the center of the cross-section, and surrounded by a cylindrical insulator of known dielectric constant, it is difficult to arrange an axial aperture in the catheter for a guide wire or filament. An improved catheter is desired.